The present invention relates generally to the field of diesel or other internal combustion engine exhaust systems and, more specifically, to an apparatus and method for reducing the release of emissions and particulate into the atmosphere; and to an apparatus and method for regenerating exhaust traps by manipulating the exhaust through-flow.
Diesel or other internal combustion engine exhaust filtering systems are used to filter particulate from engine exhaust. Particulate contains, inter alia, ash and soot. Exhaust filtering systems reduce the escape of particulate and other emissions into the atmosphere. Federal and state regulations govern the maximum amount of particulate which can be released into the atmosphere. For example, the U.S. diesel emissions regulations for the years of 1999 and 2000 limited the release of particulate from highway tracks to be less than 0.1 g/Hp-Hr. Such regulations have resulted in intensive efforts during the last twenty years to reduce the amount of particulate emitted from the diesel engine exhaust stack. The preferred approach and the one which most effort has been expended, is to reduce the amount of particulate generated by the engines. Another approach has been to filter or trap the particulate matter from the exhaust gas stream prior to its release into the atmosphere. Due to stringent emission requirements, the development and improvement of practical and reliable particulate trap systems is desirable.
To reduce the amount of particulate matter being released into the atmosphere, and to meet federal and state regulations, diesel and other internal combustion engine exhaust is normally filtered through depth-type traps or porous ceramic monolithic particulate traps. The depth-type traps employ various types of filter media including but not limited to porous or fibrous ceramics or metals. Further, some employ catalytic coatings. The catalyzed depth-type traps filter the particulate and promote its combustion. However, they are relatively expensive. These catalyst coated depth-type traps can generate undesirable emissions.
Most of the effort has been directed toward trap systems that use a porous ceramic monolith that contains many passages somewhat like a honeycomb as illustrated in Frost et al., U.S. Pat. No. 4,415,344. The honeycomb section monoliths can be extruded, are relatively inexpensive and provide a large passage surface area for a given size trap. By plugging the exit ends of alternate passages and the entrance ends of the remaining passages, the exhaust gas is forced through the porous walls of the in-flow passages into the out-flow passages. The soot particles are removed from the dirty in-flow gas and collect in a layer which builds up on the walls of the passage; clean gas exits from the out-flow passages.
While these traps remove 95-98% of the particulate from the exhaust gas stream, pressure drop across the trap builds up due to the accumulation of the soot and ash. While the soot can be burned periodically by heating all or a portion of the exhaust gas, these entail considerable loss of energy and, more seriously, the heat of combustion of the soot leads to cracking and melting of the traps.
Complicated catalyst means have been used to lower the ignition temperature of the soot to protect the trap with mixed results but burnout does not prevent longer term accumulation and pressure drop due to the buildup of incombustible ash.
Oda et al., U.S. Pat. No. 4,833,883 and Igarashi, U.S. Pat. No. 5,966,928 illustrates a cross flow monolithic ceramic trap coupled with reverse flow of high pressure air to remove the soot and ash without combustion within the particulate trap system. The through-flow passages of the cross flow trap are used to direct the dislodged soot and ash to an electrical heater outside the trap to burn the soot and collect the ash. However, this system results in loss of engine efficiency because of the amount of high pressure air (approximately 115 psi) required. In addition, the use of high pressure air requires a rather heavy structure to provide adequate strength and has also led to problems with the trap seals and valves used to control the reverse flow of air.
Yasushi et al., U.S. Pat. No. 5,941,066 and Yoshimasa et al., U.S. Pat. No. 5,930,995 illustrate depth and honeycomb wall flow particulate trap systems respectively in close proximity to the engine. The soot that is collected by these traps is burned by the heat of the engine exhaust or by electrical heaters located in each trap. By means of special passages and control valves the engine exhaust pulses are periodically employed to create back flow through the traps to dislodge and remove accumulated ash.